1. Field of the Invention
The present invention relates to a method of making a porous mold used in pressure slip casting ceramic articles.
2. Description of the Prior Art
For making most ceramic articles having a large size and complicated shape, there has been adopted for a long time a nonpressurized slip casting process using a gypsum mold. In the slip casting process, however, water is absorbed from slip into the mold by its capillary action so that the casting rate cannot be improved drastically. When the gypsum mold is saturated with the water, its capillary action is so weakened that the mold has to be dried for a long time after every one or two uses.
Therefore, a pressure casting process has recently been developed to eliminate these defects and to drastically improve productivity.
In this pressure casting process, the slip is cast under a pressure of several to 30 Kg/cm.sup.2 into a space between two mold parts, i.e., a mold cavity. The porous mold is made by filling up a space between a reinforcing, pressure-resisting iron container or a case and a base with a porous mold forming slurry or powder (e.g., a mixture of an epoxy resin and sand) and by curing the slurry or powder to form a structure integral with the reinforcing iron container or box.
According to this structure, it is remarkably difficult to make the strong pressure-resisting container or reinforcing iron box identical in shape to the product or article to be cast. Due to this difficulty, the porous layer will have a locally large thickness.
The excessive thickness of the porous layer will result in an increase of the elastic compression strain due to the slip pressure during the pressure casting to make the corners of the porous layer liable to be cracked. When the cast product is to be removed from the mold, moreover, there arises another defect in that the reaction of the compression strain causes the porous layer to bite the product, thus making the removal or demolding difficult. On the other hand, the pressure casting process is required to have not only drain passages for draining water which has been forced during the casting into the porous mold but also compressed air passages for injecting air and water into the molding surface through the porous mold when the product is to be removed from the mold. In case the mold is constructed of two upper and lower parts, for example, the upper part of the mold has to be evacuated during removal of the product from the lower mold part, so that the product may be attracted to the upper mold part but not drop. For this purpose, the air passages are also indispensable. These passages may be commonly shared and should have an interval and spacing from the molding surface to effect even injections of the water and air thereby to avoid trouble during the demolding. A variety of processes for forming such water and air passages have been proposed but encountered with difficulties in their manufacture and use.
According to one of the processes of the prior art, more specifically, a porous mold having water and air passages is made by forming a wire mesh into a cage held at a desired spacing from the molding surface, by fixing at an appropriate interval either porous tubes or tubular members made of coils covered with cloth, by fixing the cage to a pressure-resisting container at a desired spacing from the molding surface, by joining the pressure-resisting container to a base case to form a molding cavity, and by casting and curing a porous layer forming slurry in the molding cavity. This process is defective in that the cage has to be formed for each mold, in that the cage of wire mesh is difficult to have an accurate shape and to arrange the water passages accurately in the porous layer, and in that it is troublesome to make the tubular members attached to the cage. Because it is difficult to work with water and air passages having a small diameter, a portion of the passages is enlarged at the intersections. This raises problems in the strength of the mold and in the spacing from the molding surface. This thus invites trouble with the intersecting passages.
According to another process proposed (in Japanese Patent Laid-Open No. 8010/1985), flexible or rigid lines are fixed directly in a reinforcing iron box, and this iron box is joined to a base to form a mold cavity. A porous layer forming slurry is cast and cured in the mold cavity. After the slurry has cured, the lines are extracted to form draining passages. According to this process, however, the iron box cannot be identical in shape to the product or article to be formed. As a result, there arises a defect that the water and air passages cannot be formed while having their spacing from the molding surface and their interval selected, as desired.
According to still another process proposed, grooves are rormed in the rear surface of a porous mold formed in advance by means of a tool and are covered with tapes carrying an achesive to form the water and air passages. This process is followed by defects that the dimensional accuracy of the grooves is not achieved and that because of the manual work of adhering the tapes with adhesive to the grooves, the fingers of the workers will be poisoned with the adhesive. Another defect is that the compressed air will leak from the adhered portions, when the mold is used, to break the mold.